The invention relates to an assembly unit which consists of a printed circuit board with an electro-optical component and of an optical component. The invention further relates to a method of producing such an assembly unit.
In the field of integrated optics there are known assembly units in which electro-optical components such as laser diodes, LEDs and photodetectors are coupled with an optical component, for instance an optical fiber or a waveguide. For this purpose, the electro-optical components are arranged on a substrate on which the optical component is arranged, too. An example of such an assembly unit is known from the German Laid-open Document 44 10 740 in which a photodetector is coupled with an optical fiber. The photodetector and the optical fiber are received in a two-piece substrate, with a waveguide trench being provided in one substrate part, which trench is filled up by the optically transparent adhesive substance used upon bonding the two substrate parts with each other, so that a waveguide is formed which couples the photodetector and the waveguide with each other.
On producing such an assembly group there occur in particular two problems. Firstly, a precise positioning of the electro-optical component relative to the optical component can be done only with a large expenditure, for instance by using guiding slopes which are intended to provide for the optimum alignment. Examples of such configurations are known from the German Laid-open Documents 44 01 219 and 42 32 608. Secondly, there results a comparably high reject rate, because the optical member which is coupled with the electro-optical component, i.e. the waveguide in the cited example, is formed only with the final step of bonding the two substrate parts to each other. If the waveguide is defective, this will mean that also the photodetector has to be rejected, as it is inseparably linked with the assembly unit. The problem of the high reject rate is the larger, the more components have to be combined with each other. If, for example, a process consisting of 200 individual steps leads to a yield of 80%, this will mean that each individual step has to be performed with a yield of 99.9%. It is for this reason that the monolithic integrationxe2x80x94in discussion as an alternative production method and theoretically promising a very good coupling of the components usedxe2x80x94i.e. the production of electronics, opto-electronics and optics including waveguides in a single material system such as InP, for instance, will still not be utilizable for some time.
An additional problem consists in the need of achieving the required electrical terminals of the electro-optical component. When the electro-optical component is embedded in a substrate, conductors or the like have to be provided for a supply line.
From prior art it is further known to arrange electro-optical components, in particular electro-optical semiconductors, on printed circuit boards. For this, preferably the Lead Frame technology is used by means of which an electro-optical chip can be arranged on the printed circuit board with an accuracy of approximately 50 xcexcm. This accuracy, however, is not sufficient for the adjustment relative to an optical waveguide which may have dimensions in the range of down to 1 xcexcm.
The invention provides an assembly unit in which an optical component is arranged relative to an electro-optical component with the desired accuracy. The invention further provides a method for the production of such an assembly unit, which is distinguished by a high yield.
The assembly unit according to the invention consists of a printed circuit board and an optical component, the printed circuit board being provided with at least one electro-optical component, at least one conducting track for the connection of the electro-optical component, as well as a three-dimensional, microstructured adjustment formation, the electro-optical component being precisely arranged relative to the latter, and a three-dimensional positioning formation being provided on the optical component and cooperating with the adjustment formation of the printed circuit board in such a way that the optical component is precisely coupled with the electro-optical component of the printed circuit board. The assembly unit according to the invention thus consists of two subassembly units, namely first the printed circuit board with the electro-optical component and, secondly, the optical component itself. These are each operative for themselves, so that they can be checked separately from each other. So there is the possibility of checking, with regard to the printed circuit board, whether the conducting tracks for the connection of the electro-optical component, the connection of the electro-optical component with the conducting tracks and, finally, the electro-optical component itself are fully operative. With regard to the optical component, for instance a waveguide or an optical fiber, there is the possibility to check whether its function is in order. It is only upon assembly of the printed circuit board and the optical component that the electro-optical component is coupled with the optical component, namely in a passive way via the adjustment and positioning formations.
According to a preferred embodiment it is provided that the printed circuit board is an injection-molded part which is partially provided with a metallization. In this way there results an efficient manufacturing of the printed circuit board, as all formations which are required for precisely positioning the electro-optical component, for instance a microstructured depression, as well as the adjustment formation, for instance a raised adjustment cross, are able to be formed in a simple way from a suitably configured mold. The increased expenditure for the precise manufacturing has to be spent in this method only once, namely for producing the injection mold; the structures formed there with the required precision are then simply copied to the injection-molded part with the same accuracy.
According to a preferred embodiment it is provided that a cooling body is arranged at the bottom of the depression, the latter accommodating the electro-optical component. This cooling body serves the purpose to carry off the dissipated heat, in particular the heat from electro-optical transmission elements. The cooling body can consist of a metal layer, for example, which is simultaneously formed with the metallization of the printed circuit board. In this case the cooling body may be used as one of the terminals for the electro-optical component, if this terminal is connected with the cooling body in an electrically conductive way, for instance by conductive bonding.
According to the preferred embodiment it is further provided that a press fit is present between the electro-optical component and the depression of the printed circuit board. The press fit ensures the precise arrangement of the electro-optical component in the depression, without additional measures for securing the electro-optical component being necessary.
According to a preferred embodiment it is provided that the electro-optical component has a rectangular base and the depression is formed by a circular milled receiving hole, the dimensions of which are smaller than the diagonals of the base, and that four adjustment holes are provided which are assigned to the corners of the electro-optical component and whose lines of intersection with the wall of the milled receiving hole serve for precisely aligning the electro-optical component. It is by means of the milled receiving hole that the depression of the electro-optical component can be formed in a very precise way with an even base. The adjustment holes make it possible to hold the electro-optical component in the milled receiving hole so as to be precisely aligned therein.
According to a preferred embodiment it is provided that the electro-optical component is a component which emits light without preferred direction perpendicular to its surface, and the wall of the depression forms a parabolic reflector which focuses the emitted light towards the optical component. Electro-optical components radiating light without preferred direction are in particular LED chips. The light emitted from the LED chip, which is radiated outside the angle of acceptance of the associated optical component, would be lost if e.g. the parabolic reflector was not used for forming the beam. Such a parabolic reflector replaces a focusing lens which could be employed as an alternative, but also stands for a very much higher manufacturing expenditure. The reflective layer of the parabolic reflector can be formed by the metallization in a particularly simple way, this metallization being applied onto the printed circuit board for forming the conducting tracks, too.
As an optical component an optical fiber may be used, for example, the outer contour of which forms the positioning formation, and the adjustment formation of the printed circuit board and cooperating with the positioning formation may be formed by a guiding groove for the optical fiber, the fiber being received in the groove. In this embodiment of the invention, the optical component is directly aligned by means of its outer geometryxe2x80x94formed in the nature of a microstructure with the required precisionxe2x80x94on the adjustment means and, hence, relative to the electro-optical component of the printed circuit board, so that the desired electrical coupling between the electro-optical component and the optical component is obtained. The guiding groove is preferably formed with a V-shaped cross-section.
The optical component can be alternatively arranged on a substrate on which the positioning formation is formed, the latter being microstructured. In this embodiment of the invention, the optical component is indirectly aligned relative to the electro-optical component, with interposition of the substrate. This is particularly recommendable if the optical component is a waveguide or a mirror. In these cases, the optical components need the substrate quasi as a carrier structure. The waveguide is in fact usually formed in a waveguide trench in the substrate, and the mirror may be formed by a surface which is made so as to be reflective and has a suitable geometric structure, so that for instance a concave mirror is provided whichxe2x80x94in a way similar to a parabolic reflectorxe2x80x94focuses the light provided by the electro-optical component towards a further optical component, for instance the end face of an optical fiber.
A method according to the invention for producing an assembly unit of a printed circuit board on which at least one electro-optical component is arranged, and of an optical component which is coupled with the electro-optical component, comprises the following steps: a printed circuit board blank is provided, a depression for receiving the electro-optical component and a three-dimensional adjustment formation being formed; then, the printed circuit board blank is finished by it being partially metallized, so that a conducting track is formed; Subsequently, the electro-optical component is arranged in the depression and connected with the conducting track; further, an optical component is provided separately from the printed circuit board, which optical component is provided with a three-dimensional positioning formation; finally, the printed circuit board and the optical component are assembled, the adjustment and positioning formations engaging into each other and leading to a precise alignment of printed circuit board and optical component relative to each other; in a last step the printed circuit board and the optical component are secured to each other. This method allows to manufacture the assembly group in a particularly economic way, because, as is preferably provided, both the electro-optical component of the printed circuit board and the optical component can be checked in terms of their correct functioning prior to joining the printed circuit board and the optical component. In case that a correct functioning can not be determined, only the subassembly concerned has to be rejected, so that the overall reject rate of the method is considerably improved. The precise alignment of the components relative to each other, which is necessary for the coupling between the electro-optical component and the optical component, is obtained so to speak automatically in a passive way by the mutual engagement between adjustment and positioning formations.
If the printed circuit board blank is injection-molded, as this is provided according to a preferred embodiment, then the depression for receiving the electro-optical component and the three-dimensional adjustment formation are formed from the injection mold, so that they are obtained with the required precision without any further working steps.
According to a preferred embodiment it is provided that for arranging the electro-optical component in the depression, the printed circuit board is heated. The thermal expansion occurring with heating makes it possible to freely insert the component into the depression. The shrinkage occurring on cooling down results in that the electro-optical component is safely and reliably retained in the depression with an appropriate press-fit, without further steps being necessary. For arranging the electro-optical component in the depression, it is as an alternative also possible to bend the printed circuit board such that the depression enlarges towards outside. If the printed circuit board with the electro-optical component inserted therein returns to its initial position, the walls of the depression will firmly press against the electro-optical component which in this way is firmly held in the depression.
According to a preferred embodiment it is provided that the printed circuit board and the optical component are directly bonded to each other. In this way the printed circuit board and the optical component are reliably joined to each other, and the adhesive substance can be additionally used for filling up a possible free space between the corresponding faces of the electro-optical component and the optical component. The quality of the coupling between both components will be enhanced if the adhesive substance consists of a highly transparent material and completely fills up the free space in the beam path between the electro-optical component and the optical component, this, however, not being strictly necessary.
According to a preferred embodiment it is provided that the optical component is attached to a substrate on which the positioning formation is formed and which is joined to the printed circuit board. This results in a higher freedom in terms of the type of connection between the printed circuit board and the optical component. As an alternative to the bonding mentioned above it can also be provided that the printed circuit board and the substrate are soldered to each other. For this, in particular the metallization can be made use of, which is applied onto the printed circuit board, as well as a metallization which at option is applied onto the substrate and acts there, for instance, as a reflective surface in the nature of a mirror.